In electric automobiles, the load on electrically driven devices such as a motor varies considerably and frequently at startup and during acceleration, deceleration and hill climbing. Therefore, batteries mounted in the electric automobiles for driving electrically driven devices are apt to have smaller capacity and shorter life in comparison with those used under constant load.
In order to overcome these disadvantages, Patent Document 1 to be described below proposes a power supply device for use in electric automobiles, which is formed by combination of a battery for main power supply, and a battery or capacitor for auxiliary power supply, so that the auxiliary power supply bears the substantial, rapid and frequent variation in the load on the electrically driven device, whereby the variation in the load on the battery for main power supply is minimized.
The power supply system as described above is capable of collecting kinetic energy generated by the electrically driven device during deceleration and braking, in the auxiliary power supply as electric power, and supplying the collected electric power to the electrically driven device if required. The operation to collect electric power from the electrically driven device into the auxiliary power supply is referred to as a regeneration mode, and the operation to supply electric power from the auxiliary power supply to the electrically driven device is referred to as a motoring mode. A power supply system performing these operations is referred to as a hybrid power supply system. An electric automobile having such a hybrid power supply system mounted thereon is referred to as a hybrid vehicle.
FIG. 12 is a diagram for explaining a typical conventional hybrid power supply system for use in electric automobiles.
In FIG. 12, a hybrid power supply system 1 is formed by a main power supply 10, an electrically driven device 20, and an auxiliary power supply 30.
The main power supply 10 is composed of an engine 11, an electric generator 12, and an inverter 13. The electrically driven device 20 is composed of an inverter 21 and a motor 22. The inverter 13 and the inverter 21 are connected to each other through a positive line 14 and a negative line 15. A voltage V0 is applied between the positive line 14 and the negative line 15. In the following description, the section formed of the main power supply 10, the electrically driven device 20, and the positive and negative lines 14, 15 will be referred to simply as the main power supply 10 unless there is danger of misunderstanding.
The auxiliary power supply 30 is composed of a bidirectional boosting chopper 31 and an energy accumulation device 32 (hereafter, referred as the battery 32). The term “bidirectional” as used in the present invention means that electric power is reversibly transmitted from the auxiliary power supply 30 to the main power supply 10, or from the main power supply 10 to the auxiliary power supply 30, through the positive and negative lines 14, 15.
The bidirectional boosting chopper 31 is composed of two semiconductor switching elements S1, S2 and an inductor 35 having an inductance L. The semiconductor switching elements S1, S2 are each formed by an IGBT and an antiparallel diode connected in parallel as an internal or external element. For convenience of following description, the IGBT is denoted by Tr, and the antiparallel diode is denoted by D. For example, the reference symbol Tr1 means the IGBT of the semiconductor switching element S1, and D1 means the antiparallel diode of the semiconductor switching element S1.
The semiconductor switching elements S1, S2 are connected in series at a connecting point a, and one end of the inductor 35 is connected to the connecting point a. A negative terminal b of the semiconductor switching element S1 is connected to the negative line 15, and a positive terminal c of the semiconductor switching element S2 is connected to the positive line 14. The positive terminal of the battery 32 is connected to the other end of the inductor 35, and the negative terminal of the battery 32 is connected to the negative line 15. This means that the battery 32 and the main power supply 10 are connected in parallel while polarities thereof being matched.
The bidirectional boosting chopper 31 temporarily accumulates electric power of the battery 32 in the inductor 35, and the accumulated electric power can be transmitted to the main power supply 10. Conversely, the electric power in the main power supply 10 can be transmitted to the battery 32.
In electric automobiles, the voltage V0 in the main power supply 10 is generally maintained at a high voltage around 600V. On the other hand, the voltage V1 of the battery 32 in the auxiliary power supply 30 is a low voltage around 300V. Maximum current flowing through the bidirectional boosting chopper 31 is of about 600 A (when the battery voltage is 300V, the transmission electric power is 120 kW, and the ripple rate is 50%).
According to the hybrid power supply system 1 as described above, in the regeneration mode, the kinetic energy of the main power supply 10 can be converted to low-voltage electric power and charged in the battery 32 of the auxiliary power supply 30. In the motoring mode, the electric power accumulated in the battery 32 of the auxiliary power supply 30 can be converted to high-voltage electric power and supplied to the main power supply 10.
This means that, according to the hybrid power supply system 1, even if the load on the electrically driven device 20 varies considerably, rapidly, and frequently, electric power can be supplied from the auxiliary power supply 30 to supplement the electric power capacity of the main power supply 10. This makes it possible to drive the electrically driven device 20 efficiently in a constant high voltage range.
However, the hybrid power supply system 1 has four problems as described below.
(1) High Element Voltage Rating
The semiconductor switching elements S1, S2 used in the bidirectional boosting chopper 31 have high voltage rating. When the main power supply 10 has a voltage V0 of about 600V, the semiconductor switching elements S1, S2 should have a rating of 1200V to ensure safety.
(2) High Element Current Rating
Electric current (with a peak value of about 600 A) from the battery 32 flows through the respective IGBTs and antiparallel diodes of the semiconductor switching elements S1, S2 of the bidirectional boosting chopper 31. Therefore, the semiconductor switching elements S1, S2 are required to have a high current rating of about 600 A. This makes the size of the inductor 35 large.
(3) High Electric Power Loss
Turn-on power loss and reverse recovery loss of the antiparallel diode will occur since employed is hard switching in which the IGBT of one of the semiconductor switch elements S1, S2 used in the bidirectional boosting chopper 31 is turned on in the condition where the antiparallel diode of the other semiconductor switching element is conductive. Further, the semiconductor switch elements with a high voltage rating will suffer high conduction power loss and high switching power loss, causing deterioration of the electric power conversion efficiency, also due to the high voltage in the inverter circuit.
(4) Difficulty in Size Reduction and Cost Reduction
The electric current flowing through the inductor 35 is direct current, which uses only a half of the B-H curve of an iron core. Further, since the direct current flowing through the inductor 35 is excitation current itself, a core gap is required. This requires increase of the core cross-sectional area in order to obtain high inductance.
Cores with a large core cross-sectional area are expensive and difficult to reduce the size or weight. In order to solve this problem, hybrid power supply systems are known in which magnetic parts are AC-driven.
FIG. 13 is a diagram showing a hybrid power supply system employing an AC link bidirectional DC-DC converter.
In FIG. 13, a hybrid power supply system 2 is formed by a main power supply 10 (with a voltage V0 of about 600V) and an auxiliary power supply 40. The auxiliary power supply 40 is composed of an AC link bidirectional DC-DC converter 44 in which an inverter 41 and an inverter 42 are linked to each other by a transformer 43 (herein with a winding ratio of 1:2), and an energy accumulation device 46 (hereafter explained as the capacitor 46 with a rated voltage V1 of 300V).
The inverter 41 comprises four semiconductor switching elements S1, S1, S2, S2. The semiconductor switching elements are each formed by an IGBT and an antiparallel diode like those in FIG. 12.
The semiconductor switching element pair (S1, S2) located on the left side of the inverter 41 in FIG. 13 are connected in series to each other. The positive terminal of the semiconductor switching element S1 being connected to the positive terminal of the capacitor 46, and the negative terminal of the semiconductor switching element S2 is connected to the negative terminal of the capacitor 46.
On the other hand, the semiconductor switching element pair (S2, S1) located on the right side are connected in series to each other. The positive terminal of the semiconductor switching element S2 is connected to the positive terminal of the capacitor 46, and the negative terminal of the semiconductor switching element S1 is connected to the negative terminal of the capacitor 46.
The semiconductor switching element pair (S1, S1) and the semiconductor switching element pair (S2, S2) are turned on and off alternately.
Like the inverter 41, the inverter 42 comprises four semiconductor switching elements S21, S21, S22, S22. The semiconductor switching elements are each formed by an IGBT and an antiparallel diode like those in FIG. 12.
The semiconductor switching element pair (S21, S22) located on the left side in FIG. 13 are connected in series to each other. The positive terminal of the semiconductor switching element S21 is connected to the positive terminal of the main power supply 10, and the negative terminal of the semiconductor switching element S22 is connected to the negative terminal of the main power supply 10.
On the other hand, the semiconductor switching element pair (S22, S21) on the right side are connected in series to each other. The positive terminal of the semiconductor switching element S22 is connected to the positive terminal of the main power supply 10, and the negative terminal of the semiconductor switching element S21 is connected to the negative terminal of the main power supply 10.
The semiconductor switching element pair (S21, S21) and the semiconductor switching element pair (S22, S22) are turned on and off alternately.
As shown in FIG. 13, an AC terminal d1 and an AC terminal d2 of the inverter 41 are connected to each other through a coil 44 of a transformer 43 having a winding ratio of 1:2, and an AC terminal e1 and an AC terminal e2 of the inverter 42 are connected to other through a coil 45 of the transformer 43.
The transformer 43, having a leakage inductance of a fixed value L, transmits the electric power temporarily accumulated in the leakage inductance L to the auxiliary power supply 40 or the main power supply 10 by high speed switching control of the inverters.
More specifically, in the motoring mode (when electric power is transmitted from the capacitor 46 to the main power supply 10), the inverter 41 assumes a leading phase with respect to the inverter 42. At the same time, the inverter 41 transmits electric power with a high voltage that is about twice as high as the voltage V1 to the main power supply 10 through the transformer 43.
In the regeneration mode (when electric power is transmitted from the main power supply 10 to the capacitor 46) the inverter 41 assumes a delay phase with respect to the inverter 42. At the same time, the main power supply 10 transfers electric power with a low voltage that is about a half of the voltage V0 to the capacitor 46 through the transformer 43.    Patent Document 1: Japanese Patent Application Laid-Open NO. 11-146566